Dixon MR Water-Fat Separation with Bipolar Readout Correction
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Solution Overview
Problem
Dixon-type MR imaging methods face inefficiencies in scan time and image quality due to multiple repetitions and echo spacing, leading to distortions and phase errors in multi-echo acquisitions, particularly affecting water/fat separation and quantification.
Innovation Solution
A method utilizing a multi-echo acquisition with bipolar readout gradients, where echo signals are acquired with opposite readout directions for each echo time, allowing estimation and correction of amplitude and phase errors to improve water/fat separation and quantification.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If multiple repetitions with shifted readout gradients are used for Dixon imaging, then water/fat separation is improved, but scan time increases
Solution Approach 1:
The patent combines multiple echo acquisitions into a single shot by using a multi-echo readout sequence, where multiple echo signals are acquired during one RF excitation cycle. This merges what would traditionally require multiple separate repetitions into a single acquisition event, maintaining the necessary phase offset information for water/fat separation while eliminating the repeated scanning overhead.
Solution Approach 2:
The patent employs periodic bipolar readout gradients that alternate in polarity during the echo train. These periodic gradient applications create the necessary phase shifts between water and fat signals at different echo times, enabling Dixon processing without requiring multiple separate repetitions of the entire imaging sequence.
2Measurement precision
If echo spacing is increased to accommodate multiple echoes, then water/fat separation is improved, but image quality deteriorates due to blurring and loss of coverage
Solution Approach 1:
The patent uses dynamic bipolar readout gradients that switch polarity for each echo, allowing the system to adapt the gradient waveform to the specific echo timing requirements. This dynamic gradient control enables efficient use of the available echo train duration, maintaining optimal echo spacing that balances water/fat separation capability with image quality preservation.
Solution Approach 2:
The patent changes the gradient polarity parameter for each echo in the train, using alternating positive and negative bipolar gradients. This parameter modulation creates the necessary phase differences for Dixon processing while maintaining consistent echo spacing that preserves image quality, avoiding the need to increase echo spacing at the expense of image quality.
3Measurement precision
If bipolar readout gradients are used for multi-echo acquisition, then water/fat separation is improved, but phase errors and distortions increase
Solution Approach 1:
The patent incorporates phase error correction through feedback mechanisms that monitor and adjust the phase information during the multi-echo acquisition. By using the bipolar gradient pattern to create predictable phase shifts, the system can feedback-correct for residual phase errors and distortions, maintaining reliable phase accuracy for water/fat separation despite the complex gradient waveform.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach enhances the accuracy and efficiency of water/fat separation and quantification by correcting distortions and phase errors, resulting in improved image quality and reduced scan time.
Implementation Method 1
a series of bipolar readout magnetic field gradients applied during a readout period, acquiring two or more phase-encoded echo signals during each readout period
Implementation Method 2
utilize the interaction between magnetic fields and nuclear spins in order to form two-dimensional or three-dimensional images
Implementation Method 3
there is a known precessional frequency difference of hydrogen in water and fat
Data Source
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AI summary
The invention relates to a method of MR imaging of an object (10) placed in a main magnetic field B0. It is an object of the invention to provide a method that enables an improved Dixon water/fat separation in combination with a multi-echo acquisition. The method of the invention comprises the following steps: The object (10) is subjected to an imaging sequence comprising multiple segments (S 1, S2), each segment comprising at least one RF pulse followed by a phase-encoding magnetic field gradient and a series of bipolar readout magnetic field gradients applied during a readout period. Two or more phase-encoded echo signals are acquired during each readout period, each of the echo signals being associated with a different echo time resulting from the application of the series of bipolar readout magnetic field gradients. The phase-encoding and bipolar readout magnetic field gradients are controlled so as to sample a pre-determined k-space region with both positive and negative, i.e. opposite, readout directions for each echo time. Amplitude and/or phase errors are estimated from the echo signals acquired with opposite readout directions for each echo time. Finally, an MR image is reconstructed for each echo time involving correction of image distortions according to the estimated amplitude and/or phase errors. Moreover, the invention relates to an MR system (1) and to a computer program.